Revista Mexicana de Física 43, Suplemento 1 (1997) 49-57

Studying the ao and fo at BNL and JLAB

ALEX R. DZIERBA

Indiana Univervity, E/oomington, Indiana, USA

ABSTRACT.The scalar mesons, the 10(980) and the ao(980), have been receiving much attentionoí latc. The low values of their masses and widths question their assignment to the 3Po qij scalarBanet and recent Crystal Darrel results have uncovered scalar states with more appropriate rnassesand widths. Jt has been suggested that the 10(980) ami the ao(980) are J( ¡(molecules or 4-quarkstates. This talk will review recent results from BNL (Brookhaven AGS) experiment E852 inwhich the 10(980) and the ao(980) are produced in the 71"071"0ami 71"1]channels, respectively, in7r- induced reactions. Hadronproduction characteristics oC these states may reveal their nature.The ao(980) is also observed in f¡(1295) -+ 1]7r7r.At JLAB (formerly CEBAF), an experiment isunder construction to look for the rare radiative decays rP-+ 10(980h and rP-+ ao(980h. Theratio oC thesc branching ratios is also expected to provide information Oll the substructure of thesescalars. These measurements are feasible since ¡ntense photon beams will yield a high rate oí </>'S.The possibility of studying low-mass 71"71"and 71"11scattering in 1]' -+ 1]71""at BNL and JLAB willalso be discussed.

RESUMEN.Los mesones escalares 10(980) y uo(980) han recibido últimamente mucha atención.Los bajos valores de sus masas y anchuras ponen en duda su asignamiento en el noneto escalar3Po qij. Recientemente, resultados del experimento Crystal Barrel han revelado otros estadosescalares con ma5a.-c; y anchuras más apropiadas para estos a..~;ignamientos. Se ha sugerido quelos mesones 10(980) y uo(980) sean quizás moleculas J( I? o estados con cuatro quarks. En estapresentación se discuten los resultados más recientes del experimento E852 en BNL (BrookhavenAGS) donde los mesones 10(980) y ao(980) son producidos en los canales ,,071"0y "1] respec-tivamente, en reacciones inducidas por piones. Las características de estos estados en hadro-producción podrían revelar su verdadera naturaleza. El mesón ao(980) es también observadoen el decaemiento f¡ (1285) -+ '1"71".En JLAB (anterior mento CEBAF) otro experimento estáen construcción para observar dos decaemientos radiantes raros del mesón f/>; 4> --+ /0(980), Y~ --+ ao(!J80)')'. Se espera que el cociente entre los porcentajes de desintegración de estas reac-ciones provea información sobre la subestructura de estos escalares. Estas medidas serán posiblesen JLAB porque un haz de fotones muy intenso proveerá un alto n\Ímero de mesones 4>. En éstapresentación también se discutirá la posibilidad de estudiar la dispersión de 1r7r y 1r1J de baja masausando la reacción 71' --+ 7J1r1r en llNL y JLAB.

rACS: 14.40.Cs; 13.25.Jx

l. INTRODUCTION

The scalar mesons, the 10(980) and the ao(980), have been receiving much attention oflate, in particular, thcre are questions regarding thc sllbstrllcturc of these states whichare nearly equal in mass (980 MeV) aHd width ("" (j0-90 MeV). The isoscalar lo, decaysinto 7I"1rwhile the isovector ao, decays into 1}71".Although these states have historically

50 ALEX R. DZIERBA

been included iu the 3Po qij sealar nonet, the low values for their masses aud widthshave ealled these assignments into question. Moreover, results from the Crystal Barrelindieating an isosealar 10(1365) [1] and an isoveetor ao(1450) [21with masses and widthsmore appropriate for the ground state sealar qij nonet. These reeent data add to otherreasons for jettisoning the ao(980) and 10(980) from the sealar nonet and add evideneefor these states being non -qij states, for example, either qqijij or J( k moleeules. Their" eouplings also suggest that these are not qij states [3,4J. Weinstein and Isgur [5Jhave proposed a J(k molecular pieture of these states. The issue of whether these statesmix via their eommon eoupling to J(k has been examined by Aehasov and others [6)and Bames [7J. Gribov and eollaborators [8) have.also diseussed the possibility that theao(980) and 10(980) might be novel states eonstrueted of negative energy Dirae levels.

This paper diseusses an experiment that is under construetion to measure </J mesonradiative deeays into these sealar states. This experiment [9] will use the tagged photonbeam in Hall B at Jeffersou Laboratory (JLAB-formerly known as CEBAF). Data arealso presented from Brookhaven AGS experiment E852 [lO) on the hadroproduetion ofthe 10(980) and the ao(980).

2. RADIATIVE DECAYS OF THE rjJ !NTO Jo AND no

Close, Isgur and Kumano [11] how a measurement of the ratio of branehing ratios of</J radiative deeays can provide information on the makeup of these sealar states. Inthe eonventional 3Po qij pieture, we expeet (</J -t ao,)/(</J -t lo,) = O beeause of OZIsuppression. The J(k vs. qqijij pietures can be distinguished by the eleetrie dipole natureof these deeays (whieh probes the eleetric eharges of the eonstituents weighted by thedistanee from the overall cm) through the relative phases in 1 = O and 1 = I wavefunetions and through the relative spatial distributions of quarks and antiquarks. Tosurnlnanze

• 3Po qij nonet:

• J(k moleeule:

• qqijij:

ao = ~ (u Ü - d ti)} =} </J -t ao, _ Olo = sorne si; </J -t lo,

ao} 1 [ -] </J -t aO,lo = Ji (us) (üs) :1: (ds) (ds) =} </J -t lo, = I

ao} 1 [ - J </J-tao,lo =.;'i (us) (üs) :1: (ds) (ds) =} </J -t In = 9.

(1)

(2)

(3)

STUOYINGTIIE ao ANOlo AT BNLANOJLAB 51

FIGURE 1. The 670-element ¡ead glass detector to be used in CEBAF experiment E94-016 tomeasure the decay products fraIn the c/Jmesan.

The other reason measurements of the branching ratios discussed aboye is interestinghas to do with CP studies at a q, factory. A determination of the branching ratio forq, -; (ao or ¡oh will allow a determination of the branching ratio for q, -; (K~K~ orK£Kfh [11J. The latter could be an insidious background for doing CP studies sincethe radiated photon will be very low in energy (24 MeV at most) and will likely goundetected causing confusion with

(4)

3. MEASURING RARE RADIATIVE q, DECAYS AT JLAB

One of the approved experiments in Hall B at JLAB is a measurement of rare radiativedecays of the q, meson using an all-neutral detector. The reaction

-yp-;q,p (5)

will be used to produce the q,'s. The recoil protons, ernitted at angles consistent withq, production, will be detected at the first level of triggering and the all-neutral decayproducts of the q, will be detected using a ¡ead glass calorirneter shown in the drawing ofFig. 1. A drawing of the recoil pro ton detector is shown in Fig. 2.

Tite decays of tite q, iu to K k or 3" account for 98.8% of the observed 1Il0des. Listingthe energetically allowed q, radiative decays, q, -; X -y, the candidates for X include "o,TI, P, W, TI', ao(980) and the 10(980). Only the decays q, -; ,,0-y and q, -; 'l"Y had been

52 ALEXR. DZIERBA

FIGURE2. The recail pratan detector foc CEBAF experiment E94-016.

observed with branching ratios of 0.13% and 13% respectively. Although phase spacefavors the former decay mode by a factor of 2.5, the ss content of the '1 is responsible forthe latter decay being larger, instead, by a factor of ID. The decays </J-+ p, and </J-+ w,are C-violating but the current experimental limits on the branching ratios « 5%) arenot very stringent. The decay </J -+ '1', has not been observed. This mode is particularlyrelevant since it should yield information on the quarkjgluon content of the '1' [12J.

Consider the decay

In the rest frame of the </J, the energy of the radiated photon is given by

2 2m</>- mxEo = 2m</>

(6)

(7)

For example, the radiated photon in </J-+ ao, has only 40 MeV of energy, when the </J isproduced in a symmetric e+e- operating at an energy corresponding to the </J mass.

The decay given in Eq. (1) proceeds via an electric dipole (El) or magnetic dipole(MI) transition (assuming X is a scalar or pseudoscalar) and the width is given by

r '"E~ .I(X,IEI or MII</JW

The E~ factor strongly damps the width if mx = m</>.For example

(8)

E~(</J -+ '1',)E~(</J -+ '1,)

( 2 2 )3m</>- m", _ 4 . 10-3( 2 2)3mí/> - m1j

(9)

STUOYINGTlIE "o ANOJo ATBNLANOJLAB 53

TABLE1. Table of expected yields in a 30-day run for CEBAF experiment E94-016.

Decay mode BR(%) BR in al! Number A (a) A.BR Yield of

1>-+ photons 10-' of photons (%) 10-' events

71"0'"( 0.13 13 3 35 45 38,000

'1'"( 1.3 52 3 35 18.2 150,000'1' '"((b.e) 0.04 0.09 3 30 0.03 250w'"(b) 5 43 4 20 8.6 ¡5,000

"0'"( -+ '171"0'"((b) 0.10 10 5 ¡ 1 5,300Jo'"( -+ 71"071"0'"((b) 020 20 5 10 2 15,000

Ca) A;;; acceptance as defincd in the texto

(b) The measured I¡mit is used for DR.(e) For the r¡' '"'1 mode the prediction is about 15% of the currenl mcasured limit.

The following BR are also used:'1 -+ 2'"( (39%); '1' -+ 2'"( (2.2%); W ....• 71"0'"( ••••• 3'"( (8.5%).

So the more delicate the radiated photon, the more severe is the damping and the moredifficult the detection if the </Jis produced at rest in an e+e- col!ider.

At JLAB energies, photon energies of 4 to 6 GeV are possible with an intensity of5. 1071's/sec. The cross section for 11' -+ </JI'is ahout 0.5/,b, with the </J'sproduced inthe nearly forwanl direction. Tagging the recoil proton, it should he possihle to producetagged </J's,with energies clase to the incident beam energy, at arate of 30 </J's/sec (roughlyequivalent to an e+e- col!ider operating at a luminosity of 1031 cm-2. sec-I. These ratesare reasonahle for studies of rare decays with an important honus: a hoost factor of upto 12 for the radiated photons.

Tahle 1 summarizes the expected rates for a 30-day run. Extensive Monte Cariostudies have been perfoflned to est.imat.e acceptances. All photons from the decay muststrike the lead glass detector. Photons must have a minimum energy of 100 MeV andhe separated, at the detector, be at least 6 cm from other phot.ons. The acceptanceestimates take into account the presence of electromagnetic hackgrounds in the taggedbremsstrahlung heam, as well as hadronic backgrounds. It tUfl1S out that the mostinsidious background comes from </J-+ '11 -+ 371"°1but. techniques have been developedto cope with this background. It is interesting to note, that as a by-product of thisexperiment, the present poor limits on the C-violating decay, </J-+ w 1, will he great.lyimproved.

4. STUDIES WITH AN ALL-NEUTRAL DETECTOR AT BNL

Studies of the ao(980) aud the /0(980), using an al!-neutral detector, are already inprogress in E852. Data were collected in 1994 and 1995 using the multiparticle spec-trame ter (MPS) wit.h a 3000-element lead glass calorimeter [13J on a nllmber of hadronic

54 ALEX R. DZIERDA

6000

5000 A~2000

A "000'">i1500 O

'" ~ 3000 \ 1\>i I Ji •O I I t j~ 1000

/"JI 2000

~I

SOO 1000

O O

0.0 O., , .0 LO '.0 " 30 O., 'O LO '.0

."Jf" />Iw, (GfoV) Jr"K~ Mass (GrV)

(a) (b)FIGURE3. (a) The 11'011'0mass distribution for the reaction7l'-p --+ 11'°11'°11--+ 4"(11,where It,_,,1 <0.2 CeY'. (b) The 11'011'0mass distribution for the reaclion7l'-p --+ 11'°11'°11--+ 4"(11, where It,_,,1 <0.2 CeY" The S-wave projection, after a P\VA, is shown. The data are from BNL experimentE852.

reactions, induding

71'-P-771'°7l'°11 -74')'11 (10)

71'-P-771'°'111-74')'11 (11)

71'-p -711'°11'°"11-7 6')',,, (12)

The incident beam energy is 18.5 CeY. In addition to aH-neutral final states, chargedpartide detection was employed to yield data on reactions such as

7[-P -77[+7[-'111 (13)

Figure 3a shows the 7[07[0mass distribntion for reaction of Eq. (10) for momentum-transfer-squared between the incoming 7[ and the outgoing 7[7[ system, Itl, less than0.2 Cey2 The dominant peak is the 12(1275). Another striking feature of the plot isthe dip at around 1 CeY. This is also seen in the S-wave projection, after a partial waveanalysis (PWA) is performed (sec Fig. 3b). This dip in the S-wave ha.~ been observedin other experiments [141 and is understood to he due to a narrow scalar at about 1CeY in mass, interfering with a broad S-wave resonance (at about I.:l CeY?). The PWAalso shows a rapid variation in the S-wave-D-wave phase (not shown here) at this mass,consistent with a narrow resonance, the 10(980). The mass distribution and S-waveprojection, for Itl > 0.4 CeY' are shown in Figs. 4a and 4b. Note the change in characterof these distribution, compared to the low-Itl plotR of Figs. 3a and 3b, especiaHy for theregio n below the 12(1275). There is an indication, in Fig. 5b, that the dip at about 1CeY ha., turned into a humT!. This effect has also been observed [15) and iRbeing stndiedfurther in the current analysis.

'"300

no,.~ '"e

~ "O

t "O

50

O0.0 OS " "

tr'x' MasJ (GtV)

(a)

'0 "

STUDYING 'filE "O AND Jo AT BNL AND JLAB 55

2500

2000

,.~ 1500e'¡;~

1000

500

(b)FIGURE 4. (a) The ¡rorro mass distribution for the reaction ¡r- p ~ ¡ro¡ron -}o 4')'n, where1I 1T-1o 11"11" 1 > 0.4 GeV2. (b) Thc 11'011"0 mass distributioll for the reactioIl 71"-p ---+ ¡ronon ---t 41'n,where It1f-+lTlTl > 0.4 GeV2. The S-\vave projectioll, arter a P\VA, is ShOWll. Tite data are froIll13NLexperimenl E852.

Figure 5a shows the ,),,0 mass dislribulion for reaclion of Eq. (11). The uo(980) andlhe a2(1320) are dearly observed in this dislribulion. A lil lo lhe mass dislribution,IIsing a Breil- Wigner form and a cllbic backgrollnd, yic1ds masses and widlhs consislenlwilh lhe parlide dala grollp. A PWA of lhese dala is in progress.

Figure 5b shows lhe ,/,,0,,0 mass dislribulion from reaclion of Eq. (12) amI Figure5e shows lhe ",,+,,- mass dislribnlion from reaelion of Eq. (1:1). In bolh dislributions,the ,,' -+ """ appears prominently. The other peak ill both distriblltions is likc1y duelo the f¡(1285). A Dalit, plot analysis [16] indicates that the strue!ure in lhis peak,whatever it is, is consislent wilh 100% decay into "0(980)". in fact, sc1ection of eventswhich lie in this peak provides a dean source of events which can be IIsed to determinethe parameters of the ao(980), as is diseussed in Ref. 16.

5. USING THE ,,' AS A LAllORATORY FOR STUDYING 111r AND 1r1r SCATTERING

A striking feature of Figs. 5b and 5e is the prominent ,,' signal in the ')"" al\(l massspectrum. This cOllld afford the opportnnity to employ chiral perturbation theory tech-ni,!lIes to study low mass '1" and "" seattering and extraet seattering lengths. There area nllmber of advantages to IIsing ,,' deeays:

• The system is highly constrained and thlls avoids many of the combinatorie andhypothesis sorting problems inherent with reactions (10) al\(l (11) whieh increasedramatically a..."i lhe t.wo-uody Inasses approach thrcshold .

• Reactions (10) ami (11) also suffer from final state interactions with nlldeons, whentrying to extrae! seattering lengths.

56 ALEX R. OZIERBA

...

;.. 1,1 1.0

'/lr.lr" Mau (Gt'V)

"00

" ,. ,. 10

00'

l' 20 2'

1pr' MasJ (Cf'V)

1100

,OO.

,~~ OO.

~~

'0'

(a) (b) (e)FIGURE5. (a) The 1],,0 mass distribution for the reaction ,,-1' -> '1"°,, -> 4"". (b) The '1,,°,,°mass distribution for the reaction ,,-1' -> TI "0,,0,, -> 6"". (e) The '1"+"- mass distributioll forthe reaetion ,,-1' -> 1]"+"-" -> "+"-2,,,,. The data are from the BNL experiment E852.

• Reaetions (10) and (ll) are subjeet to baekgrounds in the low-mass "." regime byproeesses like .,,- P --+ '1n and .,,- p --+ W n where 1](w) --+ 3." and one the low-energypions escapes deteetion .

• Ke4 and K~4 deeays, studied at OA<I>NE/KLOE will eventually yield elean data on."." seattering lengths and will provide important input into extracting informationon '1'" seattering from 1]' --+ '1"'''' data.

Finally, photoproduetion at JLAI3 should yield a r¡eh souree of the '1' mesons forthese studies. The should rOllghly be !of the yield of <p mesons.

G. CONCLUSIONS

There are a number of important issues whieh can be and will be addressed using pho-toproduetion of the <p and '/ mesons. With the antieipated fluxes of tagged photonsat .JLAI3, there will be eopious produetion of both these states. Studies of <p deeaysshould shed light on the substrueture of the sealar states: "o and the fa, provide impor-tant information on possibly insidious baekgrounds for CP studies at <p faetories, providesome handle on the gluon eontent of the 1] and improve [imits on the C-violating deeay:<p --+ w". Oireet '1' photoproduetion eharaeteristies (e.g., Itl dependenee) eould also leadto information on the gluon eontent of the '1'. The '1', through its '1"'''' deeay, eould alsobe used as a souree of elean events for determining ."." and '1'" seattering lengths.

ACKNOWLEDGMENTS

The author is supported by the Department of Energy. The author is indebted lo hiscolleagues at Indiana University: .J. Gunter, R. Lindenlmsch, D. Rust, E. Seott. P. Smith,C. Stcffen and S. Teige.

STUDYING THE 00 AND /0 AT BNL AND JLAB 57

REFERENCES

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(co-Spokesman), Approved by CEABF's PAC-9 in January, 1995.The collaboration includesphysicists from Catholic University of America, Jefferson Lab (formerly CEBAF), Universityof Connecticut, Indiana University, University of Notre Dame, Institute for High EnergyPhysics (Serpukhov), Rensselaer Polytechnic Institute, University of Richmond, Universityof Virginia, and College of William and Mary.

10. E852 Collaboration at llrookhaven's AGS was originally approved in 1990 (S.U. Chung andA. Dzierba-co-spokesmen) and took data in 1994 and 1995. The current co-spokesmen areN.M. Cason and S.U. Chung.

11. F. Close, N. Isgur and S. Kumano, Nud. Phys. B389 (1993) 513.12. J. L. Rosner, Phys. Rev. D 27 (1983) 1101.13. ll. Brabson et al., Nud. Instrum. Methods A322 (1993) 419; R. Crittenden et al., submitted

to Nud. Instrum. Methods.14. K.L. Au, D. Morgan, and M.R. Pennington, Phys. Rev. D 35 (1987) 1033 and references

therein.15. D. Alde et al., Z. Phys. e 66 (1995) 375.16. S.Teige et al., Properties o/ the ao(980) Meson, presented at the DPF96 Meeting at the

University of Minneapolis, August, 1996.

•